In recent years, it has been revealed that formation of an ultrafine crystal layer or a nanocrystal layer in a surface layer portion of a metallic material could cause the material to have excellent characteristics that had not existed before.
The ultrafine crystal layer refers to a layer with crystal grains each having a size of from 100 nm to 1 μm, while the nanocrystal layer refers to a layer with crystal grains each having a size of not larger than 100 um. The ultrafine crystal layer has characteristics suitable for a machine component, such as its hardness higher than that of its base material and its high compressive residual stress. Similarly, the nanocrystal layer has characteristics suitable for a machine component, such as its hardness much higher than that of its base material, its difficulty of grain growth even at a high temperature and its high compressive residual stress.
It is expected that, if the ultrafine crystal layer and the nanocrystal layer could be utilized for industries, a product constituted by a metallic material could have improved strength and performance.
Thus, there are proposed various techniques for forming the ultrafine crystal layer and the nanocrystal layer (hereinafter referred to as “nanocrystal layer or the like”) in a surface layer portion of a metallic material. For example, in JP-2003-39398A, there is proposed a technique for causing a protrusion formed in a distal end surface of a metal weight, to collide with a portion of a surface of a metallic product, so as to form the nanocrystal layer or the like in the portion of the surface of the metallic product.
Further, as another conventional technique, there is a technique using a shot peening. FIG. 16 is a schematic view showing the shot peening. This shot peening is, as shown in FIG. 16, arranged to cause hard particles G such as steels and ceramics to collide with a portion of a working surface 101a of a metallic material 101 at a high velocity, by using an ejection pressure of a compressive air ejected from an ejection device 100. The collision causes the portion of the working surface 101a to be plastically deformed, and accordingly forms the nanocrystal layer or the like in the portion of the working surface 101a. 